Grade 6 sounds reasonable. As a former first violin, the music I played was probably grade 6 level around (scholastic) grade 9/10, and approached grade 8/9 conservatory level by end of high school. My seatmate was first chair, and she was far more proficient than me, maxing out conservatory grades.
If the students are able to play as a small ensemble for a few hours a week in class, it's entirely reasonable to achieve 2-3 conservatory levels in a scholastic year. An hour or two a day of solo practice will get you there, too.
Last edited by aquinas; 09/25/17 05:33 PM.